Antibodies specific for the idiotype(s) of anti-(T,G)-A--L antibodies of two different mouse strains (C57B1/10Sn and C3H.SW) have been obtained. The idiotype(s) of these two strains are distinct and the anti-(T,G)-A--L antibodies of the majority of individuals of each strain express identical or cross-reactive idiotype(s). Since the ability of mice to respond to (T,G)A--L is under the control of H-2 linked Ir genes, these anti-idiotype antibodies are being used in conjunction with immunoassay of the binding of (T,G)-A--L to anti-(T,G)-A--L antibodies as well as both in vitro and in vivo humoral response assays to evaluate the cellular and genetic control of idiotype expression as well as the role of idiotype-bearing molecules in regulation of the immune response. T lymphocytes express anti-(T,G)-A--L idiotype(s) identical to or cross-reactive with those found on anti-(T,G)-A--L antibodies of the same strain. Expression of similar or identical idiotypes is found on both IgM and IgG antibodies, and expression of these idiotypes is controlled (at least in part) by genes linked to the heavy chain allotype locus.